1. Field of the Invention
The present invention relates to an organic electroluminescence element. In particular, the invention relates to an organic electroluminescence element with high light-emission efficiency and excellent durability.
2. Description of the Related Art
An organic electroluminescence element (hereinafter, referred to as an “organic EL element” in some cases) is composed of a light-emitting layer or a plurality of functional organic layers containing a light-emitting layer, and a pair of electrodes sandwiching these layers. The organic EL element is a device for obtaining luminescence by utilizing at least either one of luminescence from excitons each of which is obtained by recombining an electron injected from a cathode with a hole injected from an anode to produce the exciton in the light-emitting layer, or luminescence from excitons of other molecules produced by energy transmission from at least one of the above-described excitons.
Heretofore, an organic EL element has been developed by using a laminate structure from integrated layers in which each layer is functionally differentiated, whereby brightness and device efficiency are remarkably improved. For example, “Science”, vol. 267, No. 3, page 1332, (1995) discloses a two-layer laminated type device obtained by laminating a hole transport layer and a light-emitting layer also functioning as an electron transport layer; a three-layer laminated type device obtained by laminating a hole transport layer, a light-emitting layer, and an electron transport layer; and a four-layer laminated type device obtained by laminating a hole transport layer, a light-emitting layer, a hole blocking layer, and an electron transport layer.
However, many problems such as improvement in light-emission efficiency, improvement in drive durability and so on still remain for putting an organic electroluminescence element to practical use. When light-emission efficiency in particular is improved, power consumption can reduced, and drive durability is also improved; accordingly, many means for improving light-emission efficiency have been disclosed. However, a light-emitting element in which light-emission efficiency is improved by means of disposing a charge blocking layer or the like generally has a disadvantage of short drive durability. On the other hand, in many cases, a light-emitting element that is excellent in drive durability has a disadvantage of low light-emission efficiency. That is, it is difficult to achieve both of improvement in light-emission efficiency and improvement in drive durability.
In order to increase light-emission efficiency of a light-emitting element, it is important that both charges injected in the light-emitting element recombine efficiently in a light-emitting layer to be converted to light. However, in general, some injected charges pass through the light-emitting element without encountering opposite charges to cause lowering of quantum yield of the light-emitting element. As a means for improving such a deterioration of quantum yield, Japanese Patent Application Laid-Open (JP-A) No. 2001-155862 discloses a means of providing a gradation in a concentration of materials which constitute a light-emitting layer in a thickness direction of the light-emitting layer. That is, in an element having a configuration wherein an electron transporting compound and a hole transporting compound are contained in a light-emitting layer, a concentration of the electron transporting compound decreases in a direction from a cathode to an anode, and a concentration of the hole transporting compound is increased in a direction from the cathode to the anode. Charges injected from the cathode proceed by hopping on the electron transporting compound. However, since the concentration of the electron transporting compound is reduced approaching the anode, movement is more restricted as charges approach the anode. Similarly, holes injected from the anode proceed by hopping on the hole transporting compound. However, since the concentration of the hole transporting compound is reduced approaching the cathode, movement is more restricted as charges approach the cathode. As a result, probability that the two charges will encounter each other is heightened, and thus, high quantum efficiency can be expected. Furthermore, speeds of both charges become slower little by little; accordingly, the recombination does not occur locally in the light-emitting layer, and excitons are generated in the entire light-emitting layer to emit light. Since the load of light emission is dispersed over the entire light-emitting layer, the drive durability can also be expected to improve.
However, the constitution of the concentration gradation described in JP-A No. 2001-155862 which includes an electron transporting material and a hole transporting material is difficult to optimize from a practical standpoint. This is because, in the configuration described in JP-A No. 2001-155862, for instance, when a host material is a hole transporting material and a light-emitting material is an electron transporting material, a region close to a cathode is constituted by the light-emitting material in an amount of 100%. It is known that, when the concentration of the light-emitting material is too high, the light-emission efficiency is extremely deteriorated. Accordingly, in many cases, the advantageous effect of the concentration gradation cannot be exerted.
On the other hand, a method of forming a gradation in which the concentration of a light-emitting material is not so high is proposed. For instance, JP-A No. 2004-6102 discloses a configuration in which a light-emitting layer is constituted by a host material and a light-emitting material, and the concentration gradation is formed so that a concentration of the light-emitting material does not become 100%. In this configuration, the light-emitting material is not contained in a high concentration, and efficiency deterioration due to concentration quenching is not caused; accordingly, the advantages due to the concentration gradation can be exerted. However, when the light-emitting material is, for instance, a hole transporting material and a host material is a material that can hardly transport holes, holes proceed by hopping on a light-emitting material and, since the concentration becomes lower proceeding in a cathode direction, are restricted in movement, but electrons of opposite polarity move by hopping on the host material and are not effectively restricted in movement. Accordingly, efficiency deterioration due to electrons passing through the light-emitting element without recombining cannot be inhibited from occurring, and further, expansion of a light emitting region is not sufficient; accordingly, improvement in durability is small.
Until now, means that sufficiently realizes the compatibility of high light-emission efficiency and high durability in a light-emitting element having a concentration gradation in a light-emitting layer has not been known.